AN ASSESSMENT OF STRENGHT PROPERTIES OF. Diti Hengchaovanich and Nimal S. Nilaweera 1

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1 ANASSESSMENTOFSTRENGHTPROPERTIESOF VETIVERGRASSROOTSINRELATIONTOSLOPE STABILIZATION DitiHengchaovanichandNimalS.Nilaweera 1 Introduction Vetivergrass(Vetiveriazizanioides)hasbeenutilizedto reducesoilerosioninmanycountriesthroughouttheworldfora longtime.itiswellunderstoodthattherootpropertiesofvetiver grass can help reduce soil erosion and strengthen slope stability when planted properly. Vetiver hedgerows cultivated across slope soil can block the passage of soil particles and developterracesbetweenthehedgesenhancingstabilityofthe slope. Some previous studies on vetiver have elucidated the morphological properties of the root and their qualitative significance for erosion control and slope stabilization (Grimshaw1994;Yoon1994).Theyemphasizetheearly developingdeeplypenetrating(sometimesupto3.5m)fibrous root system of vetiver and its capability of anchoring themselvesfirmlyintoslopesoilprofiles.however,thestrength properties of vetiver root, which also play an important role in terms of erosion control and slope stabilization by means of theirinfluencesontheshearstrengthofslopesoilhasnotyet been adequately understood. When a plant root penetrates across a potential shear surface in a soil profile, the distortion oftheshearzonedevelopstensionintheroot;thecomponent of this tension tangential to the shear zone directly resists shear, while the normal component increases the confining pressure on the shear plane. Therefore it is essential to determine tensile root strength properties in the process of

2 evaluating a plant species as a component in slope stabilization. Recently, in Malaysia the vetiver hedgerow technique starts to gain popularity in erosion control and slope stabilization. It has been and will be used to stabilize several roadembankmentsoftheeast-westhighwayandsomeother road projects. Simultaneously, attempts have been made to analyze the effects of vetiver root on slope stabilization and erosion control. This paper discusses the tensile root strength of vetiver and its contribution to soil strength through experimentsontensilerootdeterminationsandroot-permeated soil shearing, which are a part of ongoing research especially designed to access both root strength properties and morphological parameters of roots in relation to slope stability anderosioncontrol 1 EroconSdn.Bhd.,KualLumpur,Malaysia TensileRootStrengthorVitiverGrass Forthedeterminationoftensilerootstrength,matureroot specimens were sampled from two-year-old vetiver plants grownonanembankmentslope.thespecimensweretestedin freshconditionlimitingthetimeelapsedbetweenthesampling and the testing to two hours maximum. The unbranched and straight root samples, about cm long, were vertically connected to hanging spring balance via a wooden clamp at one end while the other end was fixed to a holder that was pulled down manually until the root failed. At failure, the maximum load was monitored. Subsequently, the mode of failure was examined for each sample and the results of endsheared samples and those with unusually altered rupture points were discarded. To calculate the tensile root strength, the root diameter without bark was used since the bark failed

3 before the root due to its weaker strength properties, and eventually the total tensile stress transferred to the root core. About 80 vetiver root specimens of different diameter classes varying from 0.2 to 2.2 mm were tested and the results were interpreted as the ultimate tensile force and tensile strength in relationtorootdiameterwithoutbark. The ultimate root tensile force against the root diameter plot for the vetiver roots is presented in Fig. 1. The power regression analysis of the relationship between the ultimate roottensileforceandtherootdiameterprovidesthebestfitwith thefollowingequation: F1=46.93d (1) whereisf 1 -ultimatetensilerootforce,and d-therootdiameter Thispowerregressionfunctioncanbeusedtopredictthe ultimate tensile force of a vetiver root with known diameter. A comparison of tensile resistance of vetiver roots with those of somehardwoodspeciesismadeinfig.2.theultimatetensile root forces versus root diameter relationships for Japanese ceder, Dipterocarpus alatus, and Rocky Moutain Douglas fir wereobtainedfromearlyworksofabeandiwamoto(1986), Nilaweera(1994),andBurroughsandThomas(1977) respectively. The comparison clearly indicates that the tensile resistance of vetiver roots is as high as that of the hardwood vegetation,sometimesevenhigher,contrarytothefactthatitis agrassspecies. The tensile strength of the root is defined as the ultimate tensile root force divided by the cross-sectional area of the unstressed root. If the tensile root strength is constant for vetiver roots, the ultimate tensile force, F 1, should be proportionaltod 2.AccordingtothefunctiongivenbyEquation (1),F1isproportionaltod implyingthatthetensileroot

4 strengthdecreaseswiththeincreaseinrootdiameter.figure3 illustrates the actual relationship between tensile root strength androotdiameter.thetensilerootstrength,ts,decreaseswith theincreasingrootdiameter,d,followingthepowerregression relationship. Ts=59.80d (2) Similar relationships were reported from many previous works on hardwood roots. This phenomenon implies that stronger, finer roots provide higher resistance than largerdiameter roots with comparatively low tensile strength for a givenrootcross-sectionalareaofspecies.accordingtofig.3, thetensilestrengthofvetiverrootsvariesfrom180to40mpa fortherangeofrootdiameterof mm.themeantensile strengthisabout75mpaat mmrootdiameterwhichis themostcommondiameterclassforvetiverroots.comparedto many hardwood roots, the average tensile strength of vetiver grassisveryhigh.eventhoughsomehardwoodrootsprovide highertensilestrengthvaluesthantheaveragetensilestrength of vetiver roots in the root diameter class of mm, their average tensile strength values are lower since the average rootdiameterismuchhigherthanthatofvetiverroots. DirectShearTestsonVetiverRoot-PermeatedSoil Roots of trees and other vegetation provide a reinforcing effect to soil through tensile resistance and frictional or adhesionalproperties.thereinforcingeffectortheincreaseof shear strength in soil due to roots can be quantified by conduction in-situ direct shear tests on root permeated and root-free soils at the same location. The difference between shear strength values of root-permeated soil and root-free soil sheared under the same conditions gives the shear strength increase due to the roots. In order to determine the root-

5 reinforcement effect of vetiver grass, large-scale direct shear testswereperformedonaslopedsoilprofileofanembankment vegetated with vetiver. The test apparatus comprised a shear box,ahydraulicjackingsystem,aprovingringanddialgages. Theshearboxwasmadeof8mmthicksteelplatescapableof holding firmly a soil block of 50 cm x 50 cm x 50 cm in dimensions.ahydraulicjackingsystemwithcapacityof10tons produced the shear load through the proving ring of 3 tons of measuringcapacitywhichcontrolledtheshearforcewhilefour dialgagesmeasuringthesheardisplacement(fig.4). Thetestplantswereselectedfroma50-cm-longvetiver hedgerowthatusuallyincludes3plantsplantedataspacingof 15cm.Thesoilsurroundingtheplantswasremovedleavinga 50cmx50cmx25cmroot-permeatedsoilblockcenteringthe hedgerow.subsequently,theshearboxwassetsoastocover the soil block and the loading and displacement measuring systemswereassembled.thesoilblockwith25cmheightthen sheared horizontally towards the slope direction under stresscontrolled-condition. After shearing, the shear surface and the orientation of failed roots were examined carefully in order to estimatethesheardistortionduringfailure.itwasobservedthat theaveragesheardistortionduringfailurewasabout30º.the totalcross-sectionalrootareaontheshearplaneandthebulk weight of roots in the sheared soil block were measured in order to determine the root area ratio and the biomass, respectively. This procedure was followed for each 25 cm of depthundertheentirevetiverhedgerowlengthof50cmupto 1.5m depth. For each depth level of shearing, a root-free soil profile adjacent to the root-permeated soil profile was also sheared under the same shearing conditions. Each soil block wasshearedunderitsself-weightasthenormalload.thebulk density of test soil was determined before each test for a

6 comparisonofthenormalloadontheroot-permeatedsoilblock with that on the counterpart root-free soil block. Each pair of testswasmadeunderequal,normalstressconditions. Table 1. Shear strength increase in soil profile due to root penetrationoftwo-year-oldvetiverplantswithspacing15cmin ahedgerowof50cmlength Depth DR AR Ar/A S % S S(kN/m 2 ) (m) (kg/m 3 ) (mm 2 ) X10-4 (kn/m 2 ) DR bulkweightofrootinunitsoilvolume,ar rootareaon shearsurface, A area of the shear surface, S shear strength increase in soilduetoroots, S-shearstrengthincreaseinsoildueto1cm 2 rootarea. The test results were processed in order to obtain the relationship between the shear stress and shear displacement foreachtest.figure5presentstheshear-stressversussheardisplacement curves, each plot representing the relationships forroot-permeatedandroot-freesoilsforeach0.25mofdepth up to 1.5 m of root penetration. The difference between the maximumshearstressofroot-permeatedsoilandthatofrootfree soil at a particular depth is defined as the shear strength increaseinsoilduetothepresenceofvetiverroots( S). Accordingtothetestresults,itisobviousthatthepenetrationof vetiverrootsinasoilprofileincreasestheshearstrengthofsoil significantly. For each test depth, the shear strength increase,

7 the corresponding root cross-sectional area, and the bulk root weightperunitvolumeofsoilweredeterminedandtabulatedin Table1.The svaluedecreaseswithindepthfrom8.90kn/m 2 at0.25mdepthto1.82kn/m 2 at1.50mdepthdependingon the number of roots penetrating through the shear surface. A comparison of the variation of s and the root cross-sectional area on the shear surface in given in Fig. 6 for the depth of root-penetration. The vetiver root penetration of a 2-year-old hedgerow with 15 cm plant spacing can increase the shear strengthofsoilinanadjacent25-cm-widestripby90%at0.25 m depth. At 0.5 m the shear strength increase is about 39% and is then gradually reduced to 12.5% at 1.50 m depth. The shearstrengthincreasedueto1cm 2 rootarea( s)is calculatedforeachtestdepthandpresentedintable1.the s valuevariesveryslightlywithanaverageof5.1kn/m 2 forthe analyzedroot-penetrationdepth. Discussion Theoretically,theaveragetensilestrengthofrootscanbe used to compute the shear strength increase in soil due to penetration of roots across a shear plane. The computation adapts the simple model of root-reinforced soil subjected to directshear(wu,1976).accordingtothismodel,thetensile forcethatdevelopsintherootswhenthesoilisshearedcanbe resolved into a tangential component which directly resists shear and a normal component which increases the confining stressontheshearplane.themodelsimplyassumesthatthe rootsarefullymobilizedduringshearing.themobilizedtensile resistance in the roots translates into an increase in shear strengthinthesoilasexpressedbythefollowingequation: s=tr[cosθtanφ+sinθ] (3)

8 Whereare: θ angleoesheardistortion φ angleofinternalfriction tr average tensile strength of roots per area unitofsoil Theaveragetensilestrengthofrootsperareaunitofsoil can be determined by multiplying the average tensile strength oftheroots(tr)bythefractionofthesoilcross-section occupiedbyroots,ortherootarearatio(ar/a). UsingTR=75MPa,φ=30ºandθ=30º,theincreasein theshearstrengthinthesoilduetovetiverrootwascalculated foreachtestdepthandtabulatedintable2withexperimental results. Table2.Theexperimentalandcomputed svaluesatvariable depths. Depth,m s,kn/m 2 s,kn/m 2 (experimental) (computed) Thecomputed svaluesareabout3timesashighasthe values obtained from the field experiments. The disparity between svaluesobtainedfromexperimentsandcomputation can be attributed to assumptions made on the rootreinforcementmodelandthenatureofrootspecimensusedin the tensile tests. During shearing of root-permeated soil, the

9 tensile strength of each and every root was not mobilized completely as assumed in the model. Some roots were pulled outcompletelyorpartlybyaruptureatafinerpointbelowthe shear surface providing a lower resistance to shearing than expected. Even though the root penetration of vetiver is generally vertical, as assumed in the model, some root orientationsobliquetotheshearsurfacecangiverisetolower shear-strength increase in soil. In actual conditions, the root crookedness, jointing and the presence of young roots yield lower svaluesthanthoseexpectedfromstraight,unbranched andmaturerootwhicharestrongerthantheformer. Thought the adaptation of the root reinforcement model does not compute the shear strength increase directly, an estimationofshear-strengthincreasecanbemadebydividing thecomputedvaluesbyafactorof3forvetiver-root-permeated soil with the angle of internal friction of 30º. Furthermore, the correlation of s with the root area ratio and bulk root weight perunitofsoilvolumeclearlyindicatelinearrelationshipswhich canbeusedtopredicttheshearstrengthincreaseinsoildueto vetiverroots(fig7).thevalue sincreaseslinearlywiththe root area ratio in the order of 2.7 x 10 4 for vetiver grass. The relationship between the s and bulk root weight per volume unit of soil indicates some positive intercept of shear-strength increaseowingtothenatureofrootsandrootpenetrationwith depth.atshallowerdepths,thefractionsofrootweightgivenby obliquelyorientedrootsandbytherootsterminatingbeforethe shear zone are higher than those at deeper levels. Therefore, atshallowdepthstherootweightisnotdirectlyproportionalto the s value. As a consequence, a positive intercept in the relationshipbetweenthe svalueandthebulkrootweightper area unit of soil appears though the intercept should be theoreticallyzero.foraknownroot-arearatiooraknownbulk

10 weightofrootspervolumeunitofsoil,theserelationshipscan be used to predict the s value instead of rather difficult and expensivedirectsheartests.carefullyextrudedrootsystemsof vetiverplantsbywaterjettingcanbeusedtodeterminetheroot areaandtherootweightatdifferentdepthsofrootpenetration. In the present study, the shear-strength increase in the soil by the root penetration of a vetiver hedgerow at different depthsofupto1.5mwasdeterminedfora0.5mwidestripof soil across the slope. In general, for a 1 m-wide hedgerow spacingthese svaluescanbeuseddirectlyatrelevantdepth intervals throughout the slope. However, for greater hedgerow spacings the s values should be corrected according to the pertaining areas of influence. It was unable to investigate the influenceofvetiverrootsontheshearstrengthofsoilbelow1.5 depthduetothedifficultiesencounteredduringexcavationand the setting-up of testing equipment. Field evidences indicate thatagradualandslowdecreaseinrootpenetrationwithdepth after the upper most 0.5 m where a rapid decrease in root penetration occurs. According to the trend of the s decrease withdepthitcanbepredictedthatashear-strengthincreaseof about1kn/m 2 at2mdepthbelowthevetiverhedgerowtakes place. Conclusion The tensile root strength properties of vetiver grass in association with its inherited morphological root characteristics improve the resistance of soil slopes to shallow mass stability andsurfaceerosion.thetensilestrengthofvetiverrootsisas strongas,orevenstrongerthan,thatofmanyhardwoodroots which have been proven positive for root reinforcement in soil slopes. The root tensile strength of vetiver decreases with the increase of root diameter as in the case of hardwood roots.

11 Compares to hardwood vegetation, the smeller average root diameterofvetiverfurnishesaveryhighmeantensilestrength (75MPa),indicatingthatvetivergrassismoreeffectualinroot reinforcementinsoilslopes.thepenetrationoffineandstrong vetiverrootsinasoilprofilecanincreasetheshearstrengthof soilsignificantlyatshallowdepths.theshearstrengthincrease in soil due to the root penetration of a 2-year-old vetiver hedgerowwithplantspacingof15cmvariesfrom90%at0.25 m depth to 1.25 % at 1.50 m depth. The shear-strength increase in soil due to vetiver roots can be approximated by using the average tensile root strength and the existing root areaoccupiedbyvetiverrootsonapotentialshearsurfaceata certain depth or by using the relationships of shear-strength increaseinthesoilversustheroot-arearatioorthebulkweight ofrootpervolumeunitofsoil. References Abe,K;andM.Iwamoto.1986.Anevaluationoftreerooteffect onslopestabilitybytreeroot strength.j.japaneseforest.soc.68: Burroughs., E.R. ; and Thomas, B.R Declining root strengthindouglasfirafterfellingas a factor in slope stability. Research Paper INT-190, Intermountain Forest and Range Experiment Station, US ForestService,Ogden,Utah,USA. Grimhaw, R.G Vetiver grass-its use for slope and structuralstabilizationundertropical andsemi-tropicalconditions.proc.intconf.onvegetation andslopes,oxford,england

12 Nilaweera, N.S Effects of tree roots on slope stability: ThecaseofKhaoLuangMountain area, Southern Thailand. Doctoral Dissertation, Diss. No. GT-93-2, Asian Institute of Technology, Bangkok, Thailand Yoou, P.K Important biological considerations in use of vetivergrasshedgerows(vghr) for slope protection sna stabilization. Proc. Int. Conf. on VegetationandSlopes,Oxford,England